Iran has signed a protocol that gives the UN nuclear watchdog the right to conduct snap inspections of its nuclear facilities, in a bid to disprove US claims it has a secret nuclear bomb programme.

Ali Akbar Salehi, Iran's outgoing representative to the UN International Atomic Energy Agency (IAEA), signed on behalf of Tehran. The IAEA chief, Mohamed ElBaradei, signed on behalf of the UN agency.

Iran's decision to sign the 1968 nuclear Non-Proliferation Treaty (NPT), a global pact aimed at reducing the spread of nuclear arsenals, will be greeted with relief by the international community.

Tehran has repeatedly denied that it plans to build a nuclear bomb and has been under intense pressure to sign the treaty.

The debate over whether to sign sparked heated discussions in Iran earlier this year, with hardliners saying that agreeing to the short-notice inspections the treaty permits was tantamount to allowing spies into the country.

The signing ceremony took place on schedule at the headquarters of the IAEA in Vienna at 3pm local time (1400 GMT).

"This is a long overdue but positive step forward," a Western diplomat told the Reuters news agency in anticipation of the move.

In Washington, a US state department spokesman, Richard Boucher, welcomed Iran's decision to sign but also voiced caution.

"The signature is only one step towards resolving the remaining open questions about Iran's nuclear programme and towards increasing international confidence that [it] will be limited to peaceful activities," he said.

The protocol will give the IAEA much broader inspection powers than it has under Iran's NPT Safeguards Agreement. But one analyst warned the protocol would not prevent Iran from developing the capacity to manufacture nuclear arms in case it ever wanted to "break out" of the NPT and build an atomic bomb.

"Even with the Additional Protocol, the IAEA is going to need member states to provide intelligence," Gary Samore, senior fellow at the International Institute of Strategic Studies, told Reuters. "If governments have information that Iran has not really come clean, then now is the time to give it to the IAEA